1. Field of the Invention
The invention relates to the manufacture of semi-finished products such as rolling slabs and extrusion billets using a semi-continuous aluminium alloy vertical direct chill casting process.
Specifically, the invention concerns a semi-continuous vertical casting process in which slabs or billets consisting of two or more aluminium alloys are cast simultaneously, with the aid of one or more separators.
The invention also relates to the equipment used to operate the aforementioned process and manufacture the aforementioned slabs or billets.
2. Description of Related Art
Use of aluminium in the aeronautics and automotive sectors is increasing. Applications include the manufacture of fuselage sheeting, wing spars and stringers, weight-saving body sheets and heat exchangers for the automotive industry, optical reflectors and armour plating, thermoplastic moulds, forgings and machinable parts.
In particular, such applications for aluminium, of which the above list is not exhaustive, require a compromise to be achieved between properties that are in many cases antagonistic, such as mechanical strength and workability, mechanical strength and corrosion resistance or suitability for drilling or turning.
All aluminium alloys mentioned herein are identified, unless otherwise stated, according to the designations defined in the “Registration Record Series” published regularly by the “Aluminum Association”.
Although uniform alloys may be used to fulfil certain requirements, substantial improvements are potentially achievable by, for example, controlling variations in composition between the surface and the core of a sheet, or between the surface and the core of an ingot used in an extrusion, forging or machining process, thereby differentiating between surface properties and core properties.
Cladded products, manufactured using two plates made of different alloys that are co-rolled in a hot process, exist for certain applications. Examples include:
Brazing sheets, intended primarily for heat exchangers (particularly in the automotive industry); the cladding material consists of an alloy with a lower melting point than the core, enabling it to serve as a filler material that joins the parts to be assembled during the brazing process.
Sheet for use in aircraft, in which a weakly-alloyed cladding material provides corrosion resistance for a more strongly-alloyed and mechanically stronger core. The same applies to body panels for the automotive sector, for which a weakly-alloyed cladding material is applied over a more strongly-alloyed, stronger core alloy for enhanced workability, in particular in stamping, bending and hemming operations.
The same principle also applies to a variety of other two-layer products, including optical reflectors that feature a low-cost alloy coated with a very pure aluminium alloy, and two-layer materials used in military armour.
However, this hot co-rolling process is not suitable for use with all types of alloy, particularly alloys containing significant quantities of zinc and/or magnesium (as used in the automotive, aeronautics and other industries) due to the susceptibility to surface oxidation of magnesium- and zinc-rich alloys. In addition, double hot-rolling is very often necessary, adversely impacting productivity and costs.
Accordingly, processes enabling the simultaneous casting of two alloy layers, known as bi-alloy casting, have been developed in a semi-continuous vertical casting format.
Patent application WO 03/035305 A1 and U.S. Pat. No. 7,407,713 B2 filed by Alcoa Inc., as well as other similar patents disclose the use of a separator consisting of a metal foil (unrolled from a roll) that becomes trapped in the solidification front and is entrained by the solid metal as the plate descends. This separator remains embedded in the finished slab.
A disadvantage of this solution is that it is technically challenging to implement, due in particular to the need to preheat a significant length of the metal foil, as well as issues relating to competition for space with the liquid metal supply systems, and above all, the fact that when two oxidized surfaces are introduced into the liquid metal a satisfactory metallurgical bond cannot be guaranteed, resulting in a non-negligible risk of subsequent delamination.
U.S. Pat. No. 4,567,936 filed by Kaiser Aluminum & Chemical Corporation claims a bi-alloy casting method in which the core is fully encapsulated in the coating alloy layer. This outer layer is solidified in advance and the core alloy is cast inside the casing thus formed. In this configuration, the outer alloy requires a significantly higher liquidus than the core alloy. In addition, the inner surface of the outer layer is necessarily oxidized, again making it hard to ensure a satisfactory metallurgical bond between the two layers. Furthermore, the principal claim of the aforementioned patent is to protect the Al—Li interior alloy against the effects of direct water cooling.
Patent applications US2005/0011630 A1 and US2010/0025003 A1, filed by Novelis Inc., are based on a similar idea, although the core is not fully embedded in the coating alloy. They describe a process that yields a sound interface because a temporarily-solidified layer of the inner alloy acts as the separator. This process, which is known within the industry by the name “Fusion™”, is more suitable for alloy pairs in which the outer alloy has a lower liquidus than the inner alloy. In other alloy combinations, obtaining a satisfactory metallurgical bond requires very tight control of the thermal transients. In some cases, the desired result may be impossible to achieve.
Patent application DE 44 20 697 A1 filed by the “Institut fur Verformungskunde and Hüttenmaschinen” in Leoben is based on the principle of an exogenous separator placed in close proximity to the solidification front. However, this configuration requires the separator to be positioned and maintained at a slight distance from the front, to avoid it being trapped by solidification. As a result, significant convection currents form below the separator, causing relatively pronounced mixing of the two alloys, which are therefore not truly separated.
Patent application WO 2009/024601 A1 filed by Aleris Aluminium Koblenz GmbH also claims the use of a separator, which is inserted centrally into the slab, at mid-thickness. With this process too, a mixing area forms that is hard to reproducibly control in an industrial process; in addition, the process is limited by the fact that the two layers must be the same thickness by construction. Most industrial applications require layers with very different thicknesses, however.